> æ (U+00E6) is not a ligature; it's a mostly obsolete character, with different semantics (or phonetics) than ae.

Reading that a letter in my alphabet is mostly obsolete feels really weird. No rebuttal, just a comment.

> It would never substitute æ for ae; that would misspell the word as much as substituting an o.

While that is correct, a lot of other systems actually do this exact substition. If your name contains æ it will be substituted with ae in passports, plane tickets and random other systems throughout your life.

My own username on this website is an example of a similar substition. The oe should be read as the single character ø.

I hope that the implication is that æ is obsolete in English. Because it is used in English!

Languages often simplify as they evolve, dropping "annoying" characters like æ. In fact, it was replaced by "e" (or ae itself) in most cases as the words got imported by other languages.